The present invention relates generally to the control of an electric motor load and, more particularly, to the control of an alternating current motor load through the use of controlled-current inverter circuits.
One commonly used drive system for controlling an alternating current (AC) motor employs the autosequentially commutated current source inverter which is often and more commonly called the controlled-current inverter. In this type of drive, a variable voltage, direct current (DC) source supplies a controlled variable frequency inverter circuit through a DC link circuit. One form of inverter circuit in common use today employs gate controlled rectifiers (e.g., thyristors, the most common form of which is the silicon controlled rectifier or SCR) in a bridge arrangement for polyphase use. Capacitors, across which a voltage is developed, are used to commutate (turnoff) the thyristors at appropriate times so that the current in the inverter circuit (i.e., the load current) may be transferred from one thyristor to another to provide the AC output. Some finite time is required to effect the thyristor commutation and current transfer. This time although dependent upon a number of system constants and variables as will be discussed shortly, is in a large part dependent upon the interrelationship of the capacitance of the commutating capacitors and the value of the inverter or load current.
It is known that the maximum frequency which a controlled-current inverter system can attain is related to the time required for the commutation of the current from one leg of the inverter circuit to another leg in the commutating group of the inverter. The total time to effect this commutation is the sum of two times, t.sub.1 and t.sub.2, wherein: ##EQU1## wherein:
C=Capacitance of commutating capacitor
V.sub.ci =Initial voltage on the capacitor
E.sub.m =Peak motor counter electromotive force (CEMF)
.alpha.=Phase angle of rendering thyristors conductive (180 degrees to 270 degrees for motoring)
I.sub.d =Current being commutated, and,
L=Motor leakage reactance.
From equation (2) above, it is seen that the time t.sub.2, once the commutating capacitance has been selected, is a constant for a given motor. Time t.sub.1 is a variable dependent upon the voltages and current which exist within the system as shown by equation (1). From these equations, it is seen that the frequency limit for the drive is most severe at very low or zero torque when the current to the motor is the only magnetizing current being supplied to the motor. Thus, t.sub.1 increases as the I.sub.d term in the denominator decreases and the numerator increases when .alpha. equals 270 degrees and the sin .alpha.=(-1). The remaining term in the numerator of equation (1) is the commutating capacitance C. This value is usually selected to satisfy another design criteria within the overall system, namely, the peak voltage in the circuit which is defined as: ##EQU2## In normal design, the value of C is selected at the maximum torque condition where I.sub.d is maximum to limit the value of V.sub.c to a value compatible with the power semiconductor voltage ratings and the motor insulation levels. Therefore, a drive system requiring high maximum torque and low torque at high speed presents a problem since both criteria must be satisfied by the same value (C) of the capacitor.
From the above it is seen that, basically, the larger the maximum current which must be commutated, the larger the capacitance required. Also, the larger the capacitance the greater the commutation time period. Since, however, as was indicated the capacitors are fixed once the inverter's design is fixed, it is readily seen that the system can become limited at its maximum frequency particularly during periods of light motor loads when the current is at a relatively low value. The use of larger capacitors necessary for heavy load conditions limits the operating frequency of the system particularly at light loads. Since the size of the capacitors also limits the permissible peak voltage of the system, capacitor selection normally involves some trade off between the maximum operating frequency and the peak voltages. This problem is well recognized in the industry and is explained in much greater detail in the literature. As examples, reference is made to "Transfer Function of a Controlled-Current Inverter With Purely Inductive Load" by William McMurray, Conference Record, Industry Applications Society, IEEE-IAS-1978, Annual Meeting, pages 546 to 549 and to "Commutation Modes of a Current-Source Inverter" by W. Lienau, Control in Electronics and Electrical Drives, 2nd IFAC Symposium, Copyright 1977, pages 219-299 (Pergamon Press).
In the prior art, a number of schemes have been proposed and used to increase the maximum operating frequency of a controlled current inverter drive system. In at least a majority of these cases, the net effect was to decrease the amount of time required for commutation. Known methods for achieving this function include reset circuits used to create an additional current during the commutation interval, which current does not pass through the motor. This method requires additional circuit components to handle the reset currents and an example of this method is found in U.S. Pat. No. 3,733,543 "Adjustable Frequency Current Source Power Supply of the Inverter Type" by Charles E. Rettig, issued May 15, 1973. Another method is to clamp or limit the voltage on the commutating capacitors. This permits the commutating capacitor to be relatively small to reduce the commutation time but also requires additional components to absorb the commutation energy when the capacitor voltage is clamped. It is additionally known to add an inductive load in parallel with the motor load during periods of high speed, light load operation to thus increase the inverter current and hence reduce the commutation time. Once again, additional components are required to switch the additional load into and out of the circuit at appropriate times.